Depending on the application it can be required that in particular for safety reasons a reached extension position of the piston rod relative to the cylinder is not only provided by providing the necessary pressure in the pressure cavity of the cylinder but additionally a mechanical, friction locked or form locked safety of the axial position of the connecting rod is provided, if possible at any extension position thus continuously variable or at least in individual increments with minimum distance from one increment to another.
When the operating cylinder unit is configured telescopable in several stages in mechanical position safety of this type is certainly provided at each telescope stage.
This is important in particular when the operating cylinder unit is used for lifting an object and subsequently supporting it in a lifted condition, possibly for a longer time period during which it shall be possible to switch off the pressure boosting system.
A typical application for this type of typically multi stage operating cylinder unit as a lifting device which then typically has 3 three circumferentially distributed support legs and which is therefore called a tri pod for horizontally lifting an aircraft, for example when heavy maintenance has to be performed or the landing gear has to be tested or repaired in unloaded condition.
Then different locations of the aircraft are provided with at least three operating cylinder units combined as operating cylinder devices for lifting and typically a fourth unit for support and position safety, a respective operating cylinder unit is typically installed in a so called tripod, thus to prevent tipping and including a support frame with 3 radially extending support struts. These operating cylinder units are typically also in other applications not only telescope able in one stage but often also telescope able in two or more stages when a low height of the support points for the tripods at the parked unloaded aircraft and the required maximum lifting height of the tripod.
Since wings of an aircraft are very sensitive to point loads and thus the aircraft is a very sensitive object to lift it has to be assured that vertical lifting is performed at all support points simultaneously and uniformly within very small geometric tolerances and in any case the rapid and quick loss of support at one of the support points has to be prevented since this can easily cost expensive damages to the aircraft.
In known solutions each operating cylinder unit includes a self-hemming safety thread on an outer circumference of the piston rod on which a safety nut that meshes with the safety thread can be moved axially by threading, which however has numerous disadvantages.
Typically the aircraft is then lifted up in that the piston rods are extended hydraulically from the cylinder units of each tripod very slowly and if possible synchronously after the piston rods contact the respective support point of the aircraft thus in this case the piston rods are extended in upward direction and thus at each of the support rods, e.g. the tripods an operator is placed with the only task to move the safety nut manually and continuously along the upward extending thread in a downward direction and to keep the safety nut continuously at a very small distance from a radially extending support surface arranged under the safety nut.
Additionally also an operator cannot easily walk from one tripod to another, thus from one lifting device to another and operate several of them simultaneously since the tripods are several meters tall and the safety nut is significantly above gripping height so that the operator has to stand on a platform or ladder of the tripod which he must not leave for safety reasons.
This way lifting and jacking a commercial aircraft typically takes more than an hour requiring a team of up to seven men only for manually synchronously moving the safety nuts and communicating, monitoring and providing back up and service.
Furthermore the fact that the safety thread is on an outside of the piston rod has its own disadvantages.
The outer surfaces of the thread turns on the piston rod have to contact the portion of the inner circumferential surface of the cylinder that is configured as the cylinder sealing surface in order to provide axial and lateral support of the piston rod. This way however the inner circumferential surface of the cylinder is subject to much stronger wear from the thread turns sliding along compared to a continuously smooth outer circumferential surface of a piston rod sliding along.
Omitting a friction support of the piston rod with the exterior thread relative to a portion of the inner circumferential surface of the cylinder forming a cylinder seal surface is not permissible due to a lack of tipping safety since the support portion of the piston in axial direction does not suffice for a forced support.
Another disadvantage of this external thread is a complex and expensive configuration of the friction resistant corrosion protection of the other thread which is necessary since smooth actuation of the safety nut that is continuously manually readjusted has to be provided under all conditions.
Additionally measures have to be taken to prevent a rotation of the piston rod about its axial direction since this in turn would cause a rotation relative to the contact point at the aircraft and would otherwise cause a binding of the safety nut relative to the support surface or offset of the safety nut from the support surface depending on the direction of rotation.